Vertebrates (animals with backbones) are the largest and most obvious animals on the Earth but the most numerous animals are the Invertebrates. Vertebrates are members of a single Phylum, the Chordata, which contains all the animals with spinal chords. There are about 20 Phyla in the Kingdom Animalia and all the rest are Invertebrates. Many have shells or other forms of exoskeletons, some have none at all. Their lack of internal bracing and structure limits their size but they more than make up for their small sizes by their diversities of form.
Let’s start with the smallest: Phylum Protozoa. These are the single celled animals. They far outnumber us in kinds of species and numbers of individuals and yet, before the invention of the microscope, we didn’t even know they existed. There are three main types: amoebae that move by pseudopodia; the flagellates, that use whip-like flagella to move; and the ciliates, which use rows of paddle-like hairs to row themselves about. Some Protozoa are parasitic but most are free living, thriving in ponds and streams, water tanks, lakes and puddles, and the oceans. Marine protozoa form an important part of the Zooplankton (animal drifters), which, with the single celled algae which form the Phytoplankton (plant-drifters) are the basis of all the food chains in the oceans: the food chains that sustain the great whales, the fish, the sharks and all the other vertebrates.
After the single celled animals come the Sponges; Phylum Porifera. These strange creatures look and act like plants. They are sedentary and fixed to their substrate; they look more leaf-like than animal-like, but they do not make their own food like plants. Their larval forms reveal their animal ancestry: single ciliated protozoa-like cells that settle and then grow into the adult colonies. Within the colonies each sponge cell puts out its own little cilia and feeds but they are joined together by the spongy mass of nonliving material between each cell. This protects them from single celled predators and allows them to scoop up those same organisms or their detritus and consume them. Sponges are primitive survivors which show one of the first steps to multicellularity.
Next are the colonial animals we call Phylum Cnidaria: jellyfish and anenomes and the little coral animals that are the reef-makers of the tropical oceans. Jellyfish get the largest and can kill us, so they deserve some respect. Their body form is a little more complicated than a sponge. They have a bulbous mass with tentacles hanging down, that contain specialised stinging cells from which the phylum gets its name. The great Man O War has the most fearsome reputation, but box jellyfish and the small, hard to see, Irukunji found in Australian waters are just as deadly.
The most important group of the Cnidaria are the coral animals, because they are the basis of the most diverse ecosystems on the planet. Covering only a small percentage of the oceans, coral reefs contain a huge proportion of the diversity of marine species. Coral animals are tiny colonies of cells with tentacles that live like upside down jellyfish in an apartment block with other coral animals. The apartment block is made of calcium carbonate, formed into amazing shapes, from staghorns to plate corals to brain corals. Each stalk of staghorn coral contains thousands of little residents, all busily combing the surrounding waters for detritus, algae or protozoans. There isn’t actually enough food for them in the warm, nutrient-poor, tropical waters where corals live so corals have another food source. They have a curious partnership or symbiotic relationship with the algae called Zooxanthellae. This family of primary producers live inside of the coral structures, where they make sugars by photosynthesis for their hosts and provide the wonderful colours that make the reef such a magical place to visit.
The third kind of Cnidarian is the sea anemone. It is also like an upside down jellyfish and in fact the free-living larval form is basically a little jellyfish that eventually gets the desire to turn upside down, attach itself to a rock and grow lots of tentacles. This is a successful strategy and anemones can get quite large and catch a lot of small fish. They also have stinging cells to help them and in the tropics they make use of zooxanthellae also.
Imagine a world of jellyfish and sponges. What was the next big breakthrough in Life? How about the worm? This opens a “can of worms” because the worms are not a single group and the groups are as different from one another as arthropods from echinoderms. Start with the Flatworms, Phylum Platyhelminthes. These are primitive worms with only two cell layers and no internal body cavity or coelom, but they do have a head. That was the big breakthrough for worms: they all have heads. Phylum Nematoda, Phylum Aschelminthes, Phylum Annelida, worms all have a basic form with a head at the front, a long tubular body in the middle and a pointy bit at the end. Some have segments and some don’t. Some are parasites and some are free-living, most are small though earthworms can get very long and most are underrated for their importance in the scheme of things. But it is said that if you removed everything from earth but the Nematodes, there would be nematodes left where everything was… soil nematodes, water nematodes, tree and grass nematodes, and nematodes in the guts of every other animal. It would be a ghost world filled with nematodes.
My favourite worms are the Annelids: earthworms (Oligochaeta) making the soils of the world and enriching my garden, and marine annelids (Polychaeta), making our oceans much more interesting places. I like the Polychaetes, in particular, because my first paid biology job was to study Polydora websteri, a parasitic polychaete that builds muddy burrows in the shells of oysters. These little tubeworms lead rather interesting lives, though they have little in the way of a nervous system. They build many-tunnelled burrows and when they finish sweeping an area clean of food with their two long tentacles, they extend their burrows to new pastures. They react to food voraciously and to predators just as quickly, withdrawing their vulnerable tentacles deep into their burrows. They even exhibit parental care, laying long trains of egg sacs and tirelessly cleaning and aerating them until the babies hatch and swim away.
The next Phylum includes their hosts, the oysters: Phylum Mollusca. The molluscs invented and perfected the shell and include the Univalves, with one shell (snails, limpets, abalone etc), the Bivalvia, with two shells (oyster, mussels, cockles, clams etc), and the Cephalopods (squid and octopus). Some groups have secondarily lost their shells, such as the slugs. By far the most intelligent of the Molluscs are the octopus. I once had a pet’ Blue Ringed Octopus. He was poisonous so I never touched him but he lived in a tank where I worked and I fed him small crabs. He knew I was bringing him food and would get very excited, bouncing out of the plastic tube in which he lived and blowing up his blue rings. He was pretty vicious with crabs, biting them and ripping them limb from limb. But one day I put in a crab who was too big and he ate my Ocky instead.
Which brings us to the Arthropods. This is a massive Phylum and contains far more diversity than the Chordates. The three major groups are the Crustacea, which dominate the oceans and include all manner of jointed-leg creatures that prowl the waters: prawns and crayfish, lobsters and shrimp, crabs, crabs and more crabs plus the lesser known Amphipoda, Ostracoda and Copepoda. All have a hard exoskeleton which they have to shed in order to grow and jointed legs from which the phylum gets its name. Some of the tastiest marine life are Crustaceans and they are particularly important in Antarctic food chains with Krill being the basis of life for whales, seals, fish and penguins.
On land the Arthropods are mainly represented by the Insects. The variety of Insects is bewildering and cannot be covered here. There are dozens of Insect orders from the primitive dragonflies to the Big Four: beetles (Coleoptera), flies (Diptera), moths and butterflies (Lepidoptera), and bees, ants and wasps (Hymenoptera). The Arachnids (spiders, scorpions, harvestmen and others) and the Millipedes and Centipedes are further groups of the ultra-diverse Arthropoda.
One of my favourite phyla is quite small but important from an evolutionary point of view for it represents a vital transition from Annelid worms to the Arthropods: it is the worm with legs, Phylum Onychophora. These “Velvet Worms” are found in the soils of countries descended from Gondwana. I have seen them in the highlands of New Zealand and in the thin strip of rainforest country on the eastern edge of Australia. They look at first glance like Annelids for they are segmented; but they have stubby little legs like arthropods. Their ancestors were related to the ancestors of modern Arthropods and they give us an idea what that step was like so long ago.
There are a lot of smaller Phyla as well but I will finish with the Echinoderms. Echinoderm means spiny skinned and many members of this phylum are just that: starfish and brittle stars, sea urchins and sand dollars and the strange sea cucumbers. They have given up on bilateral symmetry and have gone radial. They have tube feet to move on and rows of cilia to carry food to their central mouths. Strangely enough, different as they are, Echinoderms share a common ancestor with Chordates, which they show in their larval stages. Going back to that Can of Worms, there is one kind of worm which hasn’t got a mention yet: the upside down worm.
The animal kingdom can be divided into two huge groups depending how the original ball of cells, the blastula, develops a mouth. In the vast majority of worms and other invertebrate phyla, the first hole to develop, the blastopore, becomes the mouth. This subkingdom is called the Protostomia. In the other group, the Deuterostomes, the blastopore becomes the anus. This group includes the Echinoderms and the Protochordates, the Hemichordates and the Chordata… in other words, us. What becomes the head end of the worm in the Protostomes, becomes the tail end in the Deuterostomes. Protostomes have a dorsal gut and a ventral nerve chord. Deuterostomes such as humans have a dorsal nerve chord and a ventral gut. We came from a kind of worm still seen in the little Protochordate known as Amphioxus, an upside down worm.
An introduction must be short. It cannot begin to cover the diversity of a group as large as the Invertebrates. They are fascinating creatures and they are important in so many ways that they deserve to be noticed, not overlooked. While we are busy saving such iconic species as snow leopards, blue whales and orang-utans, we should also spare a thought for the invertebrates that they and we depend on.
